Internal combustion engines, such as gasoline engines, commonly employ a four-stroke working cycle. The four strokes may be referred to as the intake, compression, combustion (power), and exhaust strokes, which occur during two crankshaft rotations per working cycle of the engine. The working cycle may be understood to begin with the intake stroke with a piston at Top Dead Center (TDC) position, when the piston is closest to the cylinder head and farthest away from the axis of the crankshaft. A stroke may be understood to refer to a full travel of the piston from Top Dead Center (TDC) position to Bottom Dead Center (BDC) position, when the piston is furthest from the cylinder head and closest to the axis of the crankshaft.
During the intake stroke, the piston may be understood to descend from the top of a cylinder (i.e. TDC) to the bottom of the cylinder (i.e. BDC), reducing the pressure inside the cylinder. During the travel of the piston, an intake valve of the cylinder may open and a mixture of air and fuel may be introduced into the combustion chamber of the cylinder, under atmospheric or greater pressure, through an intake port. The intake valve may then close.
Thereafter, during the compression stroke, the piston may then return to the top of the cylinder, compressing the air-fuel mixture in the combustion chamber. Once the piston returns to TDC, the crankshaft will have undergone the first rotation of the working cycle.
Next, the power stroke may be understood to begin when the piston is at TDC. After igniting the compressed air-fuel mixture with an igniter, such as a spark plug, the resulting pressure from the combustion of the compressed air-fuel mixture may then force the piston back down towards BDC. This stroke is the main source of the engine's torque and power.
The compressed air-fuel mixture within the combustion chamber may be ignited by the igniter near the end of the compression stroke. Igniting the air-fuel mixture before the piston reaches TDC may allow the resulting flame to better propagate and the mixture to more fully burn soon after the piston reaches TDC.
However, if the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture may push against the piston as it is moving up during the compression stroke, causing possible engine damage. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure may occur during the combustion stroke after the piston has traveled too far down the cylinder. This often results in lost power, high emissions, and unburned fuel.
After the combustion stroke, during the exhaust stroke, the piston once again returns to TDC while an exhaust valve of the cylinder may be opened. This action may evacuate the products of combustion from the combustion chamber of the cylinder by pushing combustion products through an exhaust port. The exhaust valve may then close. Once the piston returns to TDC, the crank shaft will have undergone the second rotation of the working cycle and the engine will thereafter repeat the cycle.
In certain situations, the internal combustion engine may exhibit abnormal combustion. Abnormal combustion in a spark-initiated internal combustion engine may be understood as an uncontrolled explosion occurring in the combustion chamber as a result of ignition of combustible elements therein by a source other than the igniter.
Two types of abnormal combustion may include detonation and pre-ignition. Detonation may be understood as the spontaneous combustion of end-gas (remaining fuel-air mixture) in the combustion chamber occurring after normal combustion is initiated by the igniter. Without being bound to a particular theory, unburned end-gas, under increasing heat and pressure (from the normal progressive burning process and combustion) spontaneously combusts, ignited solely by the intense heat and pressure. The remaining fuel in the end-gas may lack sufficient octane rating to withstand the combination of heat and pressure without igniting. As a result, multiple flame fronts within the combustion chamber may form instead of a single flame kernel. Thus, detonation may be characterized by an instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the normal flame front.
If these multiple flames collide, they may do so with such force that produces a sudden rise in cylinder pressure. From a pressure trace of the combustion chamber, the normal burn would be accompanied by a normal pressure rise, followed by a sudden increase (spike) when the detonation occurs after the igniter spark. The spike in pressure may create a force in the combustion chamber which may cause the structure of the engine to ring, or resonate, much as if it were hit by a hammer, with a sharp metallic pinging/knocking noise. Resonance which is characteristic of combustion detonation may occur at about 3 to 6 kilohertz. Thus, the pinging/knocking heard may be understood to be ringing of the engine structure in response to the pressure spikes. The hammer-like shock waves created by detonation may subject various components of the engine (e.g. cylinder head, head gasket, pistons and rings) to damage.
More particularly, a rapid rise in cylinder pressure may occur due to a very rapid combustion speed (close to instantaneous) for knocking combustion. High speed pressure waves may be created that “bounce” between combustion chamber walls at high frequencies. Due to their fast speed and the high temperatures present in the combustion chamber, heat transfer rates into components such as piston may increases dramatically to where melting of piston crown material can occur, usually around edges of piston top land.
To inhibit engine knock particularly associated with detonation, an internal combustion engine may be equipped with one or more knock sensors. The knock sensor generally may include a piezoelectric element, which generates a voltage when a vibration is applied thereto. The piezoelectric element may be tuned to the frequency of engine knock and, when the piezoelectric element is exposed to this frequency, a voltage may be generated which provides a signal to an engine controller. The greater the magnitude of the frequency, the greater the voltage generally generated by the piezoelectric element. The voltage signal may then be used by the engine controller to adjust ignition spark and/or fuel injection timing until the detonation stops.
Unfortunately, in order to best detect vibrations associated with engine knock, the engine's knock sensor must generally be in close proximity to the cylinder, and multiple knock sensors may be required depending upon the number of cylinders and the engine's size. Consequently, replacement of the knock sensor(s) may be difficult and costly depending on how difficult the sensor is to replace.
Pre-ignition may be understood as an abnormal form of combustion resulting from ignition of the air-fuel mixture prior to ignition by the igniter. Anytime the air-fuel mixture in the combustion chamber is ignited prior to ignition by the igniter, such may be understood as pre-ignition.
Without being bound to a particular theory, historically pre-ignition has occurred during high speed operation of an engine when a particular point within the combustion chamber of a cylinder may become hot enough during high speed operation of the engine to effectively function as a glow plug (e.g. overheated spark plug tip, overheated burr of metal) to provide a source of ignition which causes the air-fuel mixture to ignite before ignition by the igniter. Such pre-ignition may be more commonly referred to as hot-spot pre-ignition, and may be inhibited by simply locating the hot spot and eliminating it.
More recently, vehicle manufacturers appear to have observed intermittent abnormal combustion in their production of turbocharged gasoline engines, particularly at low speeds and medium-to-high loads. More particularly, when operating the engine at speeds less than or equal to 2,000 rpm and under a load with a break mean effective pressure (BMEP) of greater than or equal to 10 bars, a condition which may be referred to as low-speed pre-ignition (LSPI) may occur in a very random and stochastic fashion. Without being bound to a particular theory, LSPI is not understood to occur at a particular location within an engine which functions as a hot-spot for pre-ignition as with hot-spot pre-ignition, and thus has proven more difficult to eliminate.
What is needed is a method and apparatus to detect and inhibit, either by reducing or preventing, the occurrence of LSPI combustion event(s) in an internal combustion engine before damage to the engine may occur.